1. Field of the Invention
The present invention relates to a process for the preparation of urea in which the gas stream released from the process for making melamine, consisting predominantly of ammonia and carbon dioxide, is recovered and, without further treatment, used in the urea process in the synthesis of urea. More particularly, the recovered gas stream is fed to a high pressure section of a urea plant.
2. Description of Related Art
Urea can be prepared by introducing ammonia and carbon dioxide into a synthesis zone at a suitable pressure, for example 12.5-35 MPa, and at a suitable temperature, for example 160-250xc2x0 C. Ammonium carbamate is formed first according to the following reaction:
2 NH3+CO2xe2x86x92H2Nxe2x80x94COxe2x80x94ONH4
Urea is subsequently formed by dehydrating the ammonium carbamate according to the following equilibrium reaction:
H2Nxe2x80x94COxe2x80x94ONH4←xe2x86x92H2Nxe2x80x94COxe2x80x94NH2+H2O
The degree to which the latter conversion takes place depends on the temperature and the ammonia excess applied, among other factors. The solution obtained as the reaction product predominantly consists of urea, water, ammonium carbamate and unbound ammonia. The ammonium carbamate and the ammonia need to be removed from the solution. Once removed, they are typically returned to the synthesis zone. The synthesis zone may include separate zones for the formation of ammonium carbamate and urea. However, these zones may also be combined in one piece of equipment.
Urea can be prepared in a conventional urea plant. A conventional high-pressure urea plant is one in which the decomposition of the ammonium carbamate that has not been converted into urea and the expulsion of the usual excess ammonia are conducted at a pressure between 1.5 and 10 MPa which is essentially lower than the pressure in the urea synthesis reactor. The synthesis reactor is conventionally operated at a temperature of about 180xc2x0 C. to about 210xc2x0 C. and at a pressure of about 18 MPa to about 30 MPa. Ammonia and carbon dioxide are directly fed to the urea reactor. The NH3/CO2 molar ratio (N/C molar ratio) in the urea synthesis is generally between about 3 and about 5 in conventional high-pressure urea processes. The unconverted reactants are recycled, after expansion, dissociation and condensation, to the urea synthesis reactor.
A variant of a conventional process for preparing urea is described in GB-A-1309275. In the described process, off-gas, also commonly referred to as waste gas, obtained in the preparation of melamine in a high-pressure melamine process is used for the synthesis of urea. The melamine off-gas consists predominantly of ammonia and carbon dioxide. The off gas stream from the gas/liquid separator of the melamine plant is transferred via a scrubber only to a low-pressure section, i.e., a low pressure first urea synthesis section. In this low-pressure section, a urea solution is prepared in an extra reactor using the ammonia and carbon dioxide originating from the melamine plant. This urea solution is subsequently compressed and transferred to a high-pressure section of the same urea plant.
The process of GB-A-1309275 suffers from a number of drawbacks. An extra reactor is required because the pressure of the off-gas stream supplied from the melamine plant is too low, even when it originates from a high-pressure melamine process, to be used directly in a conventional high-pressure urea plant. Also, one or more extra pumps are required in order to transfer the urea produced in the first low pressure urea synthesis section to the high-pressure urea synthesis section(s).
Despite these and other efforts to effectively integrate urea and melamine production facilities, there remains a need for an industrially facile and less capital intensive process for the recovery and use of off or waste gases comprised of ammonia and carbon dioxide from a high pressure melamine plant directly in a high pressure urea plant.
The present invention offers an attractive solution to these and other industry-recognized needs by effectively utilizing an off-gas stream from a high-pressure melamine process directly in a high-pressure section of a urea stripping plant.
The off-gas stream from the high-pressure melamine process consists predominantly of ammonia and carbon dioxide. Predominantly means that more than 90 wt. % of the off-gas stream consists of ammonia and carbon dioxide, preferably more than 95 wt. %. Further the off-gas stream may contain small amounts of for example melamine, urea, isocyanic acid and/or hydrogen. The NH3/CO2 molar ratio in the off-gas stream is about 2 or higher, preferably between about 2.2. and about 4.
A high pressure section of the urea stripping plant can, for example, be a urea reactor, a stripper, a carbamate condenser, an additional pre-stripper placed between the urea reactor and the stripper, a flash vessel additionally installed between the stripper and the carbamate condenser, or to pipelines between any of such equipment.
An object of the present invention concerns improving the efficiency of high pressure urea plants. This objective can be accomplished by using a virtually water-free off-gas stream consisting predominantly of ammonia and carbon dioxide obtained from a high-pressure melamine plant in a high pressure section of a urea stripping plant. This results in an increased efficiency compared to supplying a water-containing carbamate stream from the melamine plant to a urea plant.
Yet another related object is to avoid a requirement for subjecting the off-gas stream from a melamine plant to absorbing or concentrating steps before going into the urea plant. This is accomplished in the present invention because the off-gas stream can already be virtually water-free and has a sufficiently high pressure.
A still further object is to obtain enhanced energy efficiencies in the production of urea. This can be accomplished with the present invention because the extra heat released in condensing the off-gas stream from the high-pressure melamine plant can be reclaimed and used to produce additional (low pressure) steam.